Web/API/OfflineAudioContext/OfflineAudioContext

The OfflineAudioContext() constructor—part of the Web Audio API—creates and returns a new OfflineAudioContext object instance, which can then be used to render audio to an AudioBuffer rather than to an audio output device.

Syntax

var offlineAudioCtx = new OfflineAudioContext(numberOfChannels, length, sampleRate);

var offlineAudioCtx = new OfflineAudioContext(options);

Parameters

You can specify the parameters for the OfflineAudioContext() constructor as either the same set of parameters as are inputs into the AudioContext.createBuffer() method, or by passing those parameters in an options object. Either way, the individual parameters are the same.

numberOfChannels: An integer specifying the number of channels the resulting AudioBuffer should have.
length: An integer specifying the size of the buffer to create for the audio context, in sample-frames, where one sample-frame is a unit that can contain a single sample of audio data for every channel in the audio data. For example, a 5-second buffer with a sampleRate of 48000Hz would have a length of 5 * 48000 = 240000 sample-frames.
sampleRate: The sample-rate of the linear audio data in sample-frames per second. All user agents are required to support a range of 22050Hz to 96000Hz, and may support a wider range than that. The most commonly-used rate is 44100Hz, which is the sample rate used by CD audio.

It is important to note that, whereas you can create a new AudioContext using the new AudioContext() constructor with no arguments, the OfflineAudioContext() constructor requires three arguments, since it needs to create an AudioBuffer. This works in exactly the same way as when you create a new AudioBuffer with the AudioContext.createBuffer() method. For more detail, read Audio buffers: frames, samples and channels from our Basic concepts guide.

Return value

A new OfflineAudioContext object whose associated AudioBuffer is configured as requested.

Like a regular AudioContext, an OfflineAudioContext can be the target of events, therefore it implements the EventTarget interface.

Example

const offlineCtx = new OfflineAudioContext({
  numberOfChannels: 2,
  length: 44100 * 40,
  sampleRate: 44100,
});
const source = offlineCtx.createBufferSource();
// etc...

For a full working example, see our [[../../../../../../../mdn.github.io/webaudio-examples/offline-audio-context-promise/index|offline-audio-context-promise]] Github repo (see the source code too.)

Specifications

Specification	Status	Comment
Web Audio APIThe definition of 'OfflineAudioContext()' in that specification.	Working Draft

Browser compatibility

Update compatibility data on GitHub

	Desktop						Mobile
	Chrome	Edge	Firefox	Internet Explorer	Opera	Safari	Android webview	Chrome for Android	Firefox for Android	Opera for Android	Safari on iOS	Samsung Internet
`OfflineAudioContext()` constructor	Chrome Full support 55 Notes' Full support 55 Notes' Notes' Before Chrome 59, the default values were not supported.	Edge Full support ≤79	Firefox Full support 53	IE No support No	Opera Full support 42	Safari ?	WebView Android Full support 55 Notes' Full support 55 Notes' Notes' Before version 59, the default values were not supported.	Chrome Android Full support 55 Notes' Full support 55 Notes' Notes' Before Chrome 59, the default values were not supported.	Firefox Android Full support 53	Opera Android Full support 42	Safari iOS ?	Samsung Internet Android Full support 6.0 Notes' Full support 6.0 Notes' Notes' Before Samsung Internet 7.0, the default values were not supported.
Parameters accepted as a single object, as well as being passed in individually	Chrome Full support 62	Edge Full support ≤79	Firefox Full support 57	IE No support No	Opera Full support 49	Safari No support No	WebView Android Full support 62	Chrome Android Full support 62	Firefox Android Full support 57	Opera Android Full support 46	Safari iOS No support No	Samsung Internet Android Full support 8.0

Legend

Full support: Full support
No support: No support
Compatibility unknown: Compatibility unknown
See implementation notes.': See implementation notes.

OfflineAudioContext.OfflineAudioContext() by Mozilla Contributors is licensed under CC-BY-SA 2.5.